This invention relates to current transfer brushes in general and more particularly to such a brush with several parts of graphite which are combined to form a slider member and extend at least approximately perpendicular to the contact surface of the brush.
The brushes used in electric machines transfer current between a stationary machine part and a rotating machine part. They generally consist of electro graphite, natural graphite or a mixture of a metal and graphite. For, the use of graphite parts ensures good conductivity of the brush and, at the same time, good sliding properties on the contact member connected to the rotating machine part, for instance, a slip ring or a commutator.
The running properties of such a brush are determined mainly by the friction coefficient .mu. as a function of the circumferential velocity of the contact member connected to the rotating machine part and by the voltage drop .DELTA.U as a function of the current density transferred via the brush. Both quantities depend heavily on the extraneous skin formed on the rotating contact member, which skin is also called a film or patina. This extraneous skin is composed of materials of the brush and the contact member which were abraded in operation. Its thickness and nature is influenced by many factors. Thus, it is determined, for instance, by the material composition of the graphite and the contact member, by the current density provided and by the circumferential velocity and the temperature of the contact member. In addition, it depends on the contact pressure of the brush and especially also on the continuously changing effects of the atmosphere such as ground and altitude climate, air humidity and chemically agressive gases and vapors.
It has now been found that in d-c machines a maximum current density or current carrying capacity of up to about 12 A/cm.sup.2 can be obtained with such graphite brushes in continuous operation. Brushes which contain a sintered member of graphite and metal for use as the sliding member are generally provided for circumferential velocities of up to about 25 m/sec, while brushes of electro graphite are suitable for circumferential velocities of up to about 40 m/sec and brushes of natural graphite up to about 80 m/sec. The maximum velocity is generally reached only in large turbo-generators. For commutator machines, on the other hand, brushes of electro graphite are mainly used. The maximum current carrying capacity of these brushes is about 30 A/cm.sup.2 at circumferential velocities of the commutators of up to 45 m/sec.
These brushes can contain graphite member or sintered member of graphite and metal as the slider member. In addition, brushes are also known, having slider members composed of two or more layers with the same brush material, for instance, by cementing.
Since the rotating slip rings or commutators of such machines can never be made perfectly round, additional contact resistance occurs at increased circumferential velocities. This is due, in particular, to the fact that not all parts of the contact area of the slider member make uniform contact with the slip ring or the communtator. In order to meet this difficulty, a relatively high brush pressure must be generally provided, which leads to an undesirable increase of the friction coefficient .mu. of the brush and to correspondingly greater wear.